VLAN Overview 75In Figure 30 DA refers to the destination MAC address, SA refers to the sourceMAC address, and Type refers to the upper layer protocol type of the packet. IEEE802.1Q protocol defines that a 4-byte VLAN tag is encapsulated after thedestination MAC address and source MAC address to show the information aboutVLAN.Figure 31 Format of VLAN tagAs shown in Figure 31, a VLAN tag contains four fields, including the tag protocolidentifier (TPID), priority, canonical format indicator (CFI), and VLAN ID.■ TPID is a 16-bit field, indicating that this data frame is VLAN-tagged. Bydefault, it is 0x8100 in 3Com series Ethernet switches.■ Priority is a 3-bit field, referring to 802.1p priority. Refer to “QoSConfiguration” on page 299 for details.■ CFI is a 1-bit field, indicating whether the MAC address is encapsulated in thestandard format. 0 (the value of the CFI filed) indicates the MAC address isencapsulated in the standard format and 1 indicates the MAC address is notencapsulated in the standard format. The value is 0 by default.■ VLAN ID is a 12-bit field, indicating the ID of the VLAN to which this packetbelongs. It is in the range of 0 to 4,095. Generally, 0 and 4,095 is not used, sothe field is in the range of 1 to 4,094.VLAN ID identifies the VLAN to which a packet belongs. When a switch receives apacket carrying no VLAN tag, the switch encapsulates a VLAN tag with the defaultVLAN ID of the inbound port for the packet, and sends the packet to the defaultVLAN of the inbound port for transmission.MAC address learning mechanism of VLANsSwitches forward packets according to the destination MAC addresses of thepackets. So that switches maintain a table called MAC address forwarding table torecord the source MAC addresses of the received packets and the correspondingports receiving the packets for consequent packet forwarding. The process ofrecording is called MAC address learning.After VLANs are configured on a switch, the MAC address learning of the switchhas the following two modes.■ Shared VLAN learning (SVL): the switch records all the MAC address entrieslearnt by ports in all VLANs to a shared MAC address forwarding table. Packetsreceived on any port of any VLAN are forwarded according to this table.■ Independent VLAN learning (IVL): the switch maintains an independent MACaddress forwarding table for each VLAN. The source MAC address of a packetreceived on a port of a VLAN is recorded to the MAC address forwarding tableof this VLAN only, and packets received on a port of a VLAN are forwardedaccording to the VLAN’s own MAC address forwarding table.TPIDDA&SA TypePriority CFI VLAN IDVLAN Tag
